A package known as the TO-247 package is often used to package a power transistor die. A conventional TO-247 package includes three parallel extending metal leads, where the center lead extends from a center metal tab. The leads and the center tab are stamped from the same thick copper sheet. The power transistor die is mounted to a top side of the center tab. The center tab and the die are then overmolded with a plastic encapsulant such that the three leads extend parallel to one another from the amount of plastic encapsulant. A backside surface of the metal tab is, however, left exposed and is not covered with encapsulant. This backside surface of metal forms the backside of the packaged device. The backside of the tab is often electrically coupled to the bottom side of the integrated circuit die. In operation, a high voltage may be present on the bottom side of the die and the backside of the package may be fixed to a metal heatsink. For various reasons including safety reasons, it is undesirable that the high voltage from the bottom side of the die be present on the heatsink. Accordingly, insulating pads or washers are sometimes provided between the backside of the package and the heatsink to which the package is attached. These insulating pads or washers are thermally conductive, but prevent the high voltage from the backside of the package from being coupled to the heatsink. Alternatively, a thin insulating member can be provided inside the package between the bottom side of the die and top side of the metal tab. This thin insulating member is thermally conductive, but prevents the high voltage from the bottom side of the die from being coupled to the metal tab that forms the backside of the package. The package can then be connected to the heatsink without an intervening insulating pads or washers. These different structures for providing high voltage isolation are undesirable in certain circumstances of for various reasons.
U.S. Pat. No. 6,404,065 discloses a TO-247 outline package whose backside is electrically insulated from the bottom side of the die within the package. Rather than having a metal tab that extends from the center lead of the leadframe, the package involves a Direct-Bonded Copper (“DBC”) substrate. DBC substrates are also sometimes referred to as Direct Copper Bonded (“DCB”) substrates. In one example, the DBC substrate involves a substrate layer of ceramic insulator. This ceramic layer is sandwiched between a first (die-side) layer of copper and a second (back-side) layer of copper. The first die-side layer of copper is soldered to the center lead of the leadframe. The die is also die-attached to this first die-side layer of copper. After overmolding with plastic encapsulant, the second back-side layer of copper forms the backside of the package. Due to the insulating ceramic layer of the DBC substrate, the first and second layers of copper are thermally coupled to one another but the back-side copper layer is insulated from the die-side copper layer and the integrated circuit. This DBC substrate package has been in use for about ten years, and has been quite successful.